It communicates with the lead pipe in the distribution loop and both ends of the distribution loop and the outer coil, on the inner coil of the same airway. Can improve the reaction gas from the collection chamber to the upper outer coil, and on the degree of uniformity of the inner coil, so that the reaction gas in the catalyst basket balanced distribution of the heat-reactive relatively uniform, so that the temperature difference is less than 3 ° C of the same bed, improved operating conditions, to improve the utilization of the catalyst to increase the product yield, and improve economic efficiency.